Lundström, Fredrik

Abstract [en]

A new bleaching strategy with hydrogen peroxide and oxygen was investigated for the purpose of replacing the first part of the present bleaching sequence at Östrand pulp mill in order to be able to increase the closing of the bleaching liquids. Experiments with pulp collected from the Östrand pulp mill were performed to investigate how the new bleaching strategy affected oxygen delignified pulp. A simulation of the present first part was also performed. The third part of the work was to bleach the pulps to final brightness and then investigate the strength properties of the pulps. The bleaching results for the new bleaching strategy showed that delignification was improved with higher temperature and hydrogen peroxide charge in the hydrogen peroxide-stage and higher sodium hydroxide charge in the oxygen-stage. It was also found that the residence-time in the hydrogen peroxide-stage was not affecting the delignification significantly after one hour of bleaching at a high temperature. A low sodium hydroxide/hydrogen peroxide-ratio in the hydrogen peroxide-stage had a significant effect on the delignification. The comparison between the two bleaching strategies showed that the new strategy achieved a larger delignification than on presently used. The chemical cost was lower for the new strategy if the bleaching was carried out at optimal conditions and to the same extent. The result indicated that the selectivity for the present strategy was slightly higher. The final bleaching to full brightness (88% ISO) indicated that the selectivity was identical similar same for the two strategies in the final bleaching stages. The total chemical cost for this final bleaching was lower for the new strategy than for the present. The strength properties of the fully bleached pulps were very similar.